1. Field of the Invention
The present invention relates to a method for separating substances from liquids by flotation and also to an apparatus useful for the method. More particularly, the present invention relates to an improvement in a method for separating substances from liquids by flotation wherein substances present in liquids such as water are adsorbed on bubbles and separated by flotation and in an apparatus useful for such method.
2. Description of the Prior Art
In the prior art, a method wherein a gas is introduced into water to form bubbles and various kinds of substances existing in water are adsorbed on the ascending bubbles and separated from the water by flotation is known as a means for removing useful or undesirable substances present in water. Especially known hitherto in this art are, for example, a method wherein a gas is introduced in the presence of a proper capturing agent into a solution containing ionic substances such as heavy metal ions and all or a part of such ionic substances are reacted or associated with the capturing agent and adsorbed on the formed bubbles thereby separating the substance or substances from the solution (ion flotation method) and a method wherein a gas is introduced into a solution containing dissolved or suspended substances to be removed and a surfactant and then the substances are adsorbed on the formed bubbles and separated from the solution (foam separation method).
Such separation methods utilizing flotation are applied to the recovery of useful substances dissolved or suspended in liquids or to the treatment of waste water where harmful substances are removed from water. In these cases, the use of a large column is required for conducting the method on a large industrial scale. However, the use of such large column is attended with a troublesome problem in that turbulent flows including vortical and circulating flows of the liquid formed partially or integrally (from the upper part to the lower part of the column) in the column seriously reduce the separation efficiency by flotation (referred to hereinafter simply as "separation efficiency") of the substances adsorbed on the formed bubbles. When the diameter of the column is relatively small, e.g. not greater than 4-5 cm, the formation of such vortical and circulating flows can be prevented by the action of the inner wall of the column. If a larger column which is greater in the diameter is used for a large industrial scale operation, however, it is quite impossible to prevent the formation of such vortical and circulating flows of the bubbled liquid, thus reducing the separation efficiency seriously and making the operation economically unattractive. Accordingly, the use of a great number of columns having a relatively small diameter will be compelled in the treatment of a large volume of a liquid, if it is desired to maintain a practically high separation efficiency. However, the use of such a greater number of columns makes the operation extremely complicated and is not suitable for a large scale practical operation from both technical and economical viewpoints. Under these circumstances, there is a great demand for the treatment of large volumes of liquid for development of a new separation method utilizing flotation which entirely overcomes these drawbacks in the prior art.